The present invention relates in general to lasers and nonlinear optical equipment, and more particularly to laser equipment in which the energy circulating fundamental wavelength is tunable to differing output harmonic wavelengths.
Laser systems have been widely used in the medical field for the treatment of tissue. The high intensity energy of a laser beam can be concentrated into a small cross-sectional area and used to treat different types of tissues to accomplish different functions, such as cutting, cauterizing, cell destruction, etc. Each type of tissue generally reacts positively to radiation of a specific wavelength. Therefore, laser systems operating at various fundamental wavelengths are advantageous for different types of operations. For example, in ophthalmic surgical operations, it has been found that a YAG-type laser generating a wavelength of 1064 or 1320 nanometers (nm) is especially advantageous for cyclophotocoagulation or capsulotomies. Radiation wavelengths in the yellow range of the visible spectrum have been found to be advantageous in the treatment of retinal telangiectatic or intraretinal vascular abnormalities. Radiation wavelengths in the orange range of the visible spectrum have been found to be advantageous in the treatment of parafoveolar subretinal neovascularization in hypopigmented individuals. Radiation wavelengths in the red range of the visible spectrum have been found to be advantageous in the treatment of foveolar subretinal neovascularization, intraocular tumors such as choroidal malignant melanomas and retinoblastomas, as well as in the production of panretinal photocoagulation. Radiation wavelengths in the blue-green range of the visible spectrum have been found to be excellent photocoagulators.
As can be seen, different radiation wavelengths can be used to treat different physical diseases, and no single radiation band is suitable for the treatment of a wide variety of diseases. While most lasers are not monochromatic and produce radiation with a variety of wavelengths, the radiation spectrum of most lasers is relatively narrow with radiation output peaks occurring at fairly well-defined wavelength lines. However, many methods currently exist for generating additional wavelengths from existing lasers.
One technique for generating an output radiation beam having a different wavelength than that generated by the lasing medium is by the use of nonlinear frequency conversion crystals. Specialized harmonic crystals have been developed for use with currently available lasing mediums to provide an output wavelength different from the characteristic wavelength generated by the lasing medium itself. Disclosed in U.S. Pat. Nos. 3,949,323 and 4,826,283 are techniques for fabricating a harmonic crystal for use with lasing mediums where the crystal is responsive to an input fundamental wavelength to produce an output harmonic wavelength. The disclosure of these patent is incorporated herein by reference. Crystals adapted for generating harmonic wavelengths include the following types: Potassium dihydrogen phosphate (KDP or KH.sub.2 PO.sub.4), Potassium dideuterium phosphate (KD.sup.* P or KD.sub.2 PO.sub.4), Potassium titanyl phosphate (KTP or KTiOPO.sub.4), Lithium triborate (LBO or LiB.sub.3 O.sub.5), Beta-barium borate (BBO), KTA, lithium niobate doped with MgO (MgO:LiNbo.sub.3), Lithium iodate (LiIO.sub.3), RbTP, RbTA, YAB, KNbO.sub.3, Urea, BANANA crystals, and others. A more complete discussion of nonlinear devices and crystals used in such devices can be found in W. Koechner, Solid-State Laser Engineering (2d ed. 1988), which is incorporated herein by reference.
Electromagnetic waves propagating in a crystal having nonlinear optical properties induce polarization waves with frequencies that are the sum and the difference of the frequencies of the exciting waves. These polarization waves can radiate electromagnetic waves having the frequencies of the polarization waves. The energy transferred to a radiated electromagnetic wave from a polarization wave depends on the magnitude of the component of the second order polarization tensor involved because this tensor element determines the amplitude of the polarization wave and also the distance over which the polarization wave and the radiated electromagnetic wave can remain sufficiently in phase, called the coherence length. The coherence length is give by ##EQU1## wherein .DELTA.K is the difference between the wave vector of the radiated electromagnetic wave and the wave vector of the polarization wave. Phase matching occurs when the waves are completely in phase, that is when .DELTA.K=0. The condition .DELTA.K=0 can also be expressed as n.sub.3 w.sub.3 =N.sub.1 w.sub.1 .+-.n.sub.2 w.sub.2 wherein w.sub.3 =w.sub.1 .+-.w.sub.2 and where w.sub.1 and w.sub.2 are the frequencies of the incident light and w.sub.3 is that of the radiated optical wave and the n's are the corresponding refractive indices. The plus signs are appropriate when the sum frequency is the one of interest; the minus signs are appropriate when the difference frequency is the one of interest. A particular case that will be used as a simple example of nonlinear effects in second harmonic generation (SHG) where there is only one incident wave of frequency w and w.sub.1 =w.sub.2 =w and w.sub.3 =2w.
The above phase matching conditions can be met with birefringent crystals provided the refractive index difference between the ordinary and the extraordinary rays is sufficiently large to offset the change of refractive index with frequency, i.e., optical dispersion.
A complication in this phase matching process is the fact that phase matching occurs only for certain crystallographic directions. If a light ray deviates from this phase-matched direction, a mismatch occurs and .DELTA.K is no longer zero. For example, when collinear phase-matched SHG is used such a situation occurs if the alignment of the incoming beam and the phase-matched crystallographic direction is not exact or if the incoming beam is slightly divergent. In general, .DELTA.K will be a linear function of the deviation .DELTA.e from the phase-matched direction. This places a restriction on the allowable angular divergence since a useful coherence length must be maintained. In addition, because of the double refraction, the radiated electromagnetic wave and the polarization wave will in general propagate in different directions, termed "walk-off", thereby reducing the interaction distance. Phase matching under these unfavorable conditions is called "critical phase matching" (CPM). For certain crystallographic directions, .DELTA.K does not vary linearly with the angular deviation .DELTA.e, but rather varies as (.DELTA.e).sup.2. As a result, greater divergence from the phase-matched direction is allowable and no first-order "walk-off" occurs. Phase matching under these conditions is called "non-critical phase matching" (NCPM). The advantages of NCPM over CPM for practical devices are obvious. The indices of refraction can be adjusted by temperature variation or compositional variation in suitable cases so that phase matching occurs for crystallographic directions along which NCPM is possible. For biaxial crystals such as lithium triborate (LiB.sub.3 O.sub.5 or "LBO" crystals), NCPM conditions are possible for the SHG only when propagation is along certain of the principal axes of the optical indicatrix. See M. V. Hobden, J. Appl. Phys. 38, 4365 (1967).
The possibility of achieving one or more types of phase matching, and the appropriate orientation of the crystal to the incident wave depends on the existence of non-zero elements in the second order polarization tensor. Depending on the point group symmetry of the crystal, some elements will be identically zero and equalities are imposed on other elements. The magnitude of the effects will depend on the magnitude of the non-zero elements.
One laser capable of emitting radiation having a relative wide spectrum or bandwidth without the use of a harmonic crystal is the tunable dye laser. The dye laser is capable of producing moderately powered radiation emissions at any wavelength from 360 to 960 nm, depending upon the organic dye used and the lasing medium. Laser action is produced by the interaction of a pump argon laser beam with a jet of organic dye, producing laser resonance and a radiation output having a relatively broad bandwidth. A birefringent filter is used to select the desired wavelength. While dye lasers are capable of producing output radiation in the yellow, orange and red portions of the visible spectrum, dye lasers are relatively expensive and complex, increasing operating and maintenance costs.
Several other lasing mediums have proved to be inherently tunable. Such lasers include Chromium, Alexandrite, Emerald, Vanadium, Titanium, Scandium Borate, Chromium-doped Fosterite (Cr: Mg.sub.2 SiO.sub.4), Cobalt-Magnesium Fluoride (Co:MgF.sub.2), Cr:GdScGa-Garnet and Rare Earth lasers. While in most lasers, all of the energy released via stimulated emissions by the excited medium is in the form of photons, in these lasing mediums, the stimulated emissions of photons is coexistent with the emission of vibrational quanta (phonons) in a crystal lattice. Although the total energy of the lasing transition is fixed, the energy can be separated between photons and phonons in a continuous manner, resulting in broad wavelength tunability of the laser output. Specific wavelengths can be isolated using an etalon or a birefringent filter. However, the radiation wavelength of these lasers generally is greater than 700 nm, within the infrared spectrum and outside of the wavelength range that has been found most useful in medical applications.
Accordingly, a need continues to exist for a solid-state laser capable of being tuned so as to emit radiation of varying wavelengths within the visible spectrum.